The production of articles from silica particulates has customarily comprehended two very general steps:
(1) a green body is formed from the particulates; and
(2) that green body is dried and sintered (consolidated) into an integral article.
In certain operations, for example, the flame hydrolysis or oxidation of a silica-containing compound capable of being thermally decomposed into amorphous silica particulates or in the shape of an amorphous porous preform, the latter will be subsequently sintered (consolidated) into an integral body.
The sintering step in each instance has generally been carried out through simple heating, through hot pressing, and/or through hot isostatic pressing. The present invention was generally concerned with providing a method yielding a superior final product than the methods of the prior art.
Whereas the inventive method is operable with silica particulates and porous bodies no matter how produced, it is especially suitable for use with articles of high purity fused silica formed through a sol-gel process. Accordingly, the present invention will be described in specific detail as utilized in conjunction with silica articles fashioned via the latter process.
U.S. Pat. No. 4,789,389 (Schermerhorn et al.) discloses the basic method for producing fused silica glass articles of very high purity utilizing a sol-gel process. The method contemplates seven general steps:
(1) a solution is prepared which contains at least one silicon-containing organic compound having the formula Si(OR).sub.4, wherein R is an alkyl group;
(2) the silicon in the solution is polymerized to form a SiO.sub.2 gel;
(3) the gel is dried at a rate which causes the gel to fragment into granules having a mean particle size less than about 1 mm;
(4) the granules are sintered at a temperature less than about 1150.degree. C., the density of the granules after sintering being approximately equal to their maximum theoretical density;
(5) a green body is formed from the sintered granules;
(6) the green body is dried and partially sintered in a chamber by (a) raising the temperature of the chamber to above about 1000.degree. C., and (b) introducing chlorine gas into the chamber and/or subjecting the chamber to a vacuum and/or purging the chamber with an inert gas; and then
(7) the green body is fully sintered in a chamber by raising the temperature of the chamber to above about 1720.degree. C. while purging the chamber with helium or applying a vacuum to the chamber.
It is observed that, during the final sintering step, some voids and/or seeds were retained within the structure. U.S. Pat. No. 4,961,767 (Schermerhorn et al.), which patent was a division of U.S. Pat. No. 4,789,389, supra, teaches a further step of consolidating the sintered body via hot isostatic pressing in a chamber operating at a temperature above about 1150.degree. C. with an inert gas being introduced into the chamber at a pressure above about 100 psig (.about.0.69 MPa).
In practicing this further step, argon has been customarily employed as the inert gas at pressures in excess of 1000 psig (.about.6.9 MPa). Whereas the hot isostatic pressing functions to reduce the number and size of the voids and/or seeds in the final product, argon has been found to penetrate the surface of the glass body up to about 0.25" (6.35 mm), causing a green fluorescing peripheral region plus creating increased birefringence and poor refractive index homogeneity. To overcome this problem, the outer fluorescing region is removed through grinding and polishing, thereby resulting in loss of glass. That loss, coupled with the additional step of hot isostatic pressing, quite obviously increase the cost of the product.
As was observed above, the overall primary objective of the present invention was to devise a method for reducing the size of, preferably totalling eliminating the internal defects, such as seeds and voids, developed in articles of fused silica prepared from silica particulates or porous silica bodies.
A very specific objective was to devise such a method which could be utilized in the manufacture of fused silica glass articles wherein the initial step in the production of those articles involves preparing silica particulates through a sol-gel process.